Various properties are required for a metal wire used as a cord component. For example, from the viewpoint of recent environmental problems, it is particularly essential to reduce the weight of tires which contribute to increase in fuel-efficiency in automobiles. For this purpose, it is necessary that a cord used as reinforcement of a tire be highly strengthened so as to reduce the amount of use thereof.
As a method of highly strengthening a cord, it is useful to highly strengthen the wire per se which constitutes the cord. In this strengthening of wire, the composition thereof has been adjusted and/or drawing has been worked out for a metal wire material which is the starting material for a wire obtained by drawing. This has achieved to highly strengthen a cord; however, it is also a problem that the ductility of the metal wire material decreases with strengthening.
Conventionally, as a means for recovering the ductility of a metal wire material, it is common that a heat treatment at a low temperature for a short period of time, so-called bluing treatment, is performed on the metal wire material. It is tried to recover the ductility by this bluing treatment on the metal wire material.
In Patent Documents 1 and 2, for example, it was reported that the breaking elongation of the steel cord can be increased by performing a bluing treatment at a temperature range of around 400° C. for a certain retention time on a steel cord having the tensile strength of not more than 3,000 MPa.
Further, Patent Document 3 reported that the elastic elongation can be increased by performing a drawing treatment, plating treatment, and bluing treatment at a temperature range between 340° C. and 500° C. for several seconds to several tens of seconds on a steel wire.
Further, in Patent Document 4, it was reported that the ductility may be increased by performing a bluing treatment in which the retention time is adjusted to between 6 seconds and 15 minutes at a temperature range between 250° C. and 440° C. on a carbon steel wire to control the maximum value of internal friction of the carbon steel wire to within a suitable range at a temperature range between 180° C. and 220° C.
Moreover, in Patent Document 5, a correlation between the existence of an exothermic peak around 100° C. and the occurrence of delamination in torsional deformation of extra-fine high-carbon steel wire was discovered based on the analytical results of differential scanning calorimeter curve for extra-fine high-carbon steel wire and it was disclosed that decrease in ductility of strain aging (C diffusion-induced) can be suppressed by processing at a low temperature in a drawing process.
Furthermore, in Patent Document 6, it was reported that, when a heat treatment is performed at a temperature range of 250-400° C. on a metal wire material having a tensile strength of 4,000 MPa or greater, the ductility can be recovered without losing the tensile strength and bending strength of the metal wire material after the heat treatment by controlling the retention time at a temperature range so that the Fe diffusion length is within a prescribed range for the metal wire material after a heat treatment.